<!doctype html>
<html>
<head>
<meta charset='UTF-8'><meta name='viewport' content='width=device-width initial-scale=1'>

<link href='https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext' rel='stylesheet' type='text/css' /><style type='text/css'>html {overflow-x: initial !important;}:root { --bg-color:#ffffff; --text-color:#333333; --select-text-bg-color:#B5D6FC; --select-text-font-color:auto; --monospace:"Lucida Console",Consolas,"Courier",monospace; --title-bar-height:20px; }
.mac-os-11 { --title-bar-height:28px; }
html { font-size: 14px; background-color: var(--bg-color); color: var(--text-color); font-family: "Helvetica Neue", Helvetica, Arial, sans-serif; -webkit-font-smoothing: antialiased; }
body { margin: 0px; padding: 0px; height: auto; bottom: 0px; top: 0px; left: 0px; right: 0px; font-size: 1rem; line-height: 1.42857; overflow-x: hidden; background: inherit; tab-size: 4; }
iframe { margin: auto; }
a.url { word-break: break-all; }
a:active, a:hover { outline: 0px; }
.in-text-selection, ::selection { text-shadow: none; background: var(--select-text-bg-color); color: var(--select-text-font-color); }
#write { margin: 0px auto; height: auto; width: inherit; word-break: normal; overflow-wrap: break-word; position: relative; white-space: normal; overflow-x: visible; padding-top: 36px; }
#write.first-line-indent p { text-indent: 2em; }
#write.first-line-indent li p, #write.first-line-indent p * { text-indent: 0px; }
#write.first-line-indent li { margin-left: 2em; }
.for-image #write { padding-left: 8px; padding-right: 8px; }
body.typora-export { padding-left: 30px; padding-right: 30px; }
.typora-export .footnote-line, .typora-export li, .typora-export p { white-space: pre-wrap; }
.typora-export .task-list-item input { pointer-events: none; }
@media screen and (max-width: 500px) {
  body.typora-export { padding-left: 0px; padding-right: 0px; }
  #write { padding-left: 20px; padding-right: 20px; }
  .CodeMirror-sizer { margin-left: 0px !important; }
  .CodeMirror-gutters { display: none !important; }
}
#write li > figure:last-child { margin-bottom: 0.5rem; }
#write ol, #write ul { position: relative; }
img { max-width: 100%; vertical-align: middle; image-orientation: from-image; }
button, input, select, textarea { color: inherit; font: inherit; }
input[type="checkbox"], input[type="radio"] { line-height: normal; padding: 0px; }
*, ::after, ::before { box-sizing: border-box; }
#write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p, #write pre { width: inherit; }
#write h1, #write h2, #write h3, #write h4, #write h5, #write h6, #write p { position: relative; }
p { line-height: inherit; }
h1, h2, h3, h4, h5, h6 { break-after: avoid-page; break-inside: avoid; orphans: 4; }
p { orphans: 4; }
h1 { font-size: 2rem; }
h2 { font-size: 1.8rem; }
h3 { font-size: 1.6rem; }
h4 { font-size: 1.4rem; }
h5 { font-size: 1.2rem; }
h6 { font-size: 1rem; }
.md-math-block, .md-rawblock, h1, h2, h3, h4, h5, h6, p { margin-top: 1rem; margin-bottom: 1rem; }
.hidden { display: none; }
.md-blockmeta { color: rgb(204, 204, 204); font-weight: 700; font-style: italic; }
a { cursor: pointer; }
sup.md-footnote { padding: 2px 4px; background-color: rgba(238, 238, 238, 0.7); color: rgb(85, 85, 85); border-radius: 4px; cursor: pointer; }
sup.md-footnote a, sup.md-footnote a:hover { color: inherit; text-transform: inherit; text-decoration: inherit; }
#write input[type="checkbox"] { cursor: pointer; width: inherit; height: inherit; }
figure { overflow-x: auto; margin: 1.2em 0px; max-width: calc(100% + 16px); padding: 0px; }
figure > table { margin: 0px; }
tr { break-inside: avoid; break-after: auto; }
thead { display: table-header-group; }
table { border-collapse: collapse; border-spacing: 0px; width: 100%; overflow: auto; break-inside: auto; text-align: left; }
table.md-table td { min-width: 32px; }
.CodeMirror-gutters { border-right: 0px; background-color: inherit; }
.CodeMirror-linenumber { user-select: none; }
.CodeMirror { text-align: left; }
.CodeMirror-placeholder { opacity: 0.3; }
.CodeMirror pre { padding: 0px 4px; }
.CodeMirror-lines { padding: 0px; }
div.hr:focus { cursor: none; }
#write pre { white-space: pre-wrap; }
#write.fences-no-line-wrapping pre { white-space: pre; }
#write pre.ty-contain-cm { white-space: normal; }
.CodeMirror-gutters { margin-right: 4px; }
.md-fences { font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; overflow: visible; white-space: pre; background: inherit; position: relative !important; }
.md-fences-adv-panel { width: 100%; margin-top: 10px; text-align: center; padding-top: 0px; padding-bottom: 8px; overflow-x: auto; }
#write .md-fences.mock-cm { white-space: pre-wrap; }
.md-fences.md-fences-with-lineno { padding-left: 0px; }
#write.fences-no-line-wrapping .md-fences.mock-cm { white-space: pre; overflow-x: auto; }
.md-fences.mock-cm.md-fences-with-lineno { padding-left: 8px; }
.CodeMirror-line, twitterwidget { break-inside: avoid; }
.footnotes { opacity: 0.8; font-size: 0.9rem; margin-top: 1em; margin-bottom: 1em; }
.footnotes + .footnotes { margin-top: 0px; }
.md-reset { margin: 0px; padding: 0px; border: 0px; outline: 0px; vertical-align: top; background: 0px 0px; text-decoration: none; text-shadow: none; float: none; position: static; width: auto; height: auto; white-space: nowrap; cursor: inherit; -webkit-tap-highlight-color: transparent; line-height: normal; font-weight: 400; text-align: left; box-sizing: content-box; direction: ltr; }
li div { padding-top: 0px; }
blockquote { margin: 1rem 0px; }
li .mathjax-block, li p { margin: 0.5rem 0px; }
li blockquote { margin: 1rem 0px; }
li { margin: 0px; position: relative; }
blockquote > :last-child { margin-bottom: 0px; }
blockquote > :first-child, li > :first-child { margin-top: 0px; }
.footnotes-area { color: rgb(136, 136, 136); margin-top: 0.714rem; padding-bottom: 0.143rem; white-space: normal; }
#write .footnote-line { white-space: pre-wrap; }
@media print {
  body, html { border: 1px solid transparent; height: 99%; break-after: avoid; break-before: avoid; font-variant-ligatures: no-common-ligatures; }
  #write { margin-top: 0px; padding-top: 0px; border-color: transparent !important; }
  .typora-export * { -webkit-print-color-adjust: exact; }
  .typora-export #write { break-after: avoid; }
  .typora-export #write::after { height: 0px; }
  .is-mac table { break-inside: avoid; }
  .typora-export-show-outline .typora-export-sidebar { display: none; }
}
.footnote-line { margin-top: 0.714em; font-size: 0.7em; }
a img, img a { cursor: pointer; }
pre.md-meta-block { font-size: 0.8rem; min-height: 0.8rem; white-space: pre-wrap; background: rgb(204, 204, 204); display: block; overflow-x: hidden; }
p > .md-image:only-child:not(.md-img-error) img, p > img:only-child { display: block; margin: auto; }
#write.first-line-indent p > .md-image:only-child:not(.md-img-error) img { left: -2em; position: relative; }
p > .md-image:only-child { display: inline-block; width: 100%; }
#write .MathJax_Display { margin: 0.8em 0px 0px; }
.md-math-block { width: 100%; }
.md-math-block:not(:empty)::after { display: none; }
.MathJax_ref { fill: currentcolor; }
[contenteditable="true"]:active, [contenteditable="true"]:focus, [contenteditable="false"]:active, [contenteditable="false"]:focus { outline: 0px; box-shadow: none; }
.md-task-list-item { position: relative; list-style-type: none; }
.task-list-item.md-task-list-item { padding-left: 0px; }
.md-task-list-item > input { position: absolute; top: 0px; left: 0px; margin-left: -1.2em; margin-top: calc(1em - 10px); border: none; }
.math { font-size: 1rem; }
.md-toc { min-height: 3.58rem; position: relative; font-size: 0.9rem; border-radius: 10px; }
.md-toc-content { position: relative; margin-left: 0px; }
.md-toc-content::after, .md-toc::after { display: none; }
.md-toc-item { display: block; color: rgb(65, 131, 196); }
.md-toc-item a { text-decoration: none; }
.md-toc-inner:hover { text-decoration: underline; }
.md-toc-inner { display: inline-block; cursor: pointer; }
.md-toc-h1 .md-toc-inner { margin-left: 0px; font-weight: 700; }
.md-toc-h2 .md-toc-inner { margin-left: 2em; }
.md-toc-h3 .md-toc-inner { margin-left: 4em; }
.md-toc-h4 .md-toc-inner { margin-left: 6em; }
.md-toc-h5 .md-toc-inner { margin-left: 8em; }
.md-toc-h6 .md-toc-inner { margin-left: 10em; }
@media screen and (max-width: 48em) {
  .md-toc-h3 .md-toc-inner { margin-left: 3.5em; }
  .md-toc-h4 .md-toc-inner { margin-left: 5em; }
  .md-toc-h5 .md-toc-inner { margin-left: 6.5em; }
  .md-toc-h6 .md-toc-inner { margin-left: 8em; }
}
a.md-toc-inner { font-size: inherit; font-style: inherit; font-weight: inherit; line-height: inherit; }
.footnote-line a:not(.reversefootnote) { color: inherit; }
.md-attr { display: none; }
.md-fn-count::after { content: "."; }
code, pre, samp, tt { font-family: var(--monospace); }
kbd { margin: 0px 0.1em; padding: 0.1em 0.6em; font-size: 0.8em; color: rgb(36, 39, 41); background: rgb(255, 255, 255); border: 1px solid rgb(173, 179, 185); border-radius: 3px; box-shadow: rgba(12, 13, 14, 0.2) 0px 1px 0px, rgb(255, 255, 255) 0px 0px 0px 2px inset; white-space: nowrap; vertical-align: middle; }
.md-comment { color: rgb(162, 127, 3); opacity: 0.8; font-family: var(--monospace); }
code { text-align: left; vertical-align: initial; }
a.md-print-anchor { white-space: pre !important; border-width: initial !important; border-style: none !important; border-color: initial !important; display: inline-block !important; position: absolute !important; width: 1px !important; right: 0px !important; outline: 0px !important; background: 0px 0px !important; text-decoration: initial !important; text-shadow: initial !important; }
.md-inline-math .MathJax_SVG .noError { display: none !important; }
.html-for-mac .inline-math-svg .MathJax_SVG { vertical-align: 0.2px; }
.md-fences-math .MathJax_SVG_Display, .md-math-block .MathJax_SVG_Display { text-align: center; margin: 0px; position: relative; text-indent: 0px; max-width: none; max-height: none; min-height: 0px; min-width: 100%; width: auto; overflow-y: visible; display: block !important; }
.MathJax_SVG_Display, .md-inline-math .MathJax_SVG_Display { width: auto; margin: inherit; display: inline-block !important; }
.MathJax_SVG .MJX-monospace { font-family: var(--monospace); }
.MathJax_SVG .MJX-sans-serif { font-family: sans-serif; }
.MathJax_SVG { display: inline; font-style: normal; font-weight: 400; line-height: normal; text-indent: 0px; text-align: left; text-transform: none; letter-spacing: normal; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; padding: 0px; margin: 0px; zoom: 90%; }
#math-inline-preview-content { zoom: 1.1; }
.MathJax_SVG * { transition: none 0s ease 0s; }
.MathJax_SVG_Display svg { vertical-align: middle !important; margin-bottom: 0px !important; margin-top: 0px !important; }
.os-windows.monocolor-emoji .md-emoji { font-family: "Segoe UI Symbol", sans-serif; }
.md-diagram-panel > svg { max-width: 100%; }
[lang="flow"] svg, [lang="mermaid"] svg { max-width: 100%; height: auto; }
[lang="mermaid"] .node text { font-size: 1rem; }
table tr th { border-bottom: 0px; }
video { max-width: 100%; display: block; margin: 0px auto; }
iframe { max-width: 100%; width: 100%; border: none; }
.highlight td, .highlight tr { border: 0px; }
mark { background: rgb(255, 255, 0); color: rgb(0, 0, 0); }
.md-html-inline .md-plain, .md-html-inline strong, mark .md-inline-math, mark strong { color: inherit; }
.md-expand mark .md-meta { opacity: 0.3 !important; }
mark .md-meta { color: rgb(0, 0, 0); }
@media print {
  .typora-export h1, .typora-export h2, .typora-export h3, .typora-export h4, .typora-export h5, .typora-export h6 { break-inside: avoid; }
}
.md-diagram-panel .messageText { stroke: none !important; }
.md-diagram-panel .start-state { fill: var(--node-fill); }
.md-diagram-panel .edgeLabel rect { opacity: 1 !important; }
.md-require-zoom-fix foreignobject { font-size: var(--mermaid-font-zoom); }
.md-fences.md-fences-math { font-size: 1em; }
.md-fences-math .MathJax_SVG_Display { margin-top: 8px; cursor: default; }
.md-fences-advanced:not(.md-focus) { padding: 0px; white-space: nowrap; border: 0px; }
.md-fences-advanced:not(.md-focus) { background: inherit; }
.typora-export-show-outline .typora-export-content { max-width: 1440px; margin: auto; display: flex; flex-direction: row; }
.typora-export-sidebar { width: 300px; font-size: 0.8rem; margin-top: 80px; margin-right: 18px; }
.typora-export-show-outline #write { --webkit-flex:2; flex: 2 1 0%; }
.typora-export-sidebar .outline-content { position: fixed; top: 0px; max-height: 100%; overflow: hidden auto; padding-bottom: 30px; padding-top: 60px; width: 300px; }
@media screen and (max-width: 1024px) {
  .typora-export-sidebar, .typora-export-sidebar .outline-content { width: 240px; }
}
@media screen and (max-width: 800px) {
  .typora-export-sidebar { display: none; }
}
.outline-content li, .outline-content ul { margin-left: 0px; margin-right: 0px; padding-left: 0px; padding-right: 0px; list-style: none; }
.outline-content ul { margin-top: 0px; margin-bottom: 0px; }
.outline-content strong { font-weight: 400; }
.outline-expander { width: 1rem; height: 1.42857rem; position: relative; display: table-cell; vertical-align: middle; cursor: pointer; padding-left: 4px; }
.outline-expander::before { content: ""; position: relative; font-family: Ionicons; display: inline-block; font-size: 8px; vertical-align: middle; }
.outline-item { padding-top: 3px; padding-bottom: 3px; cursor: pointer; }
.outline-expander:hover::before { content: ""; }
.outline-h1 > .outline-item { padding-left: 0px; }
.outline-h2 > .outline-item { padding-left: 1em; }
.outline-h3 > .outline-item { padding-left: 2em; }
.outline-h4 > .outline-item { padding-left: 3em; }
.outline-h5 > .outline-item { padding-left: 4em; }
.outline-h6 > .outline-item { padding-left: 5em; }
.outline-label { cursor: pointer; display: table-cell; vertical-align: middle; text-decoration: none; color: inherit; }
.outline-label:hover { text-decoration: underline; }
.outline-item:hover { border-color: rgb(245, 245, 245); background-color: var(--item-hover-bg-color); }
.outline-item:hover { margin-left: -28px; margin-right: -28px; border-left: 28px solid transparent; border-right: 28px solid transparent; }
.outline-item-single .outline-expander::before, .outline-item-single .outline-expander:hover::before { display: none; }
.outline-item-open > .outline-item > .outline-expander::before { content: ""; }
.outline-children { display: none; }
.info-panel-tab-wrapper { display: none; }
.outline-item-open > .outline-children { display: block; }
.typora-export .outline-item { padding-top: 1px; padding-bottom: 1px; }
.typora-export .outline-item:hover { margin-right: -8px; border-right: 8px solid transparent; }
.typora-export .outline-expander::before { content: "+"; font-family: inherit; top: -1px; }
.typora-export .outline-expander:hover::before, .typora-export .outline-item-open > .outline-item > .outline-expander::before { content: "−"; }
.typora-export-collapse-outline .outline-children { display: none; }
.typora-export-collapse-outline .outline-item-open > .outline-children, .typora-export-no-collapse-outline .outline-children { display: block; }
.typora-export-no-collapse-outline .outline-expander::before { content: "" !important; }
.typora-export-show-outline .outline-item-active > .outline-item .outline-label { font-weight: 700; }


.CodeMirror { height: auto; }
.CodeMirror.cm-s-inner { background: inherit; }
.CodeMirror-scroll { overflow: auto hidden; z-index: 3; }
.CodeMirror-gutter-filler, .CodeMirror-scrollbar-filler { background-color: rgb(255, 255, 255); }
.CodeMirror-gutters { border-right: 1px solid rgb(221, 221, 221); background: inherit; white-space: nowrap; }
.CodeMirror-linenumber { padding: 0px 3px 0px 5px; text-align: right; color: rgb(153, 153, 153); }
.cm-s-inner .cm-keyword { color: rgb(119, 0, 136); }
.cm-s-inner .cm-atom, .cm-s-inner.cm-atom { color: rgb(34, 17, 153); }
.cm-s-inner .cm-number { color: rgb(17, 102, 68); }
.cm-s-inner .cm-def { color: rgb(0, 0, 255); }
.cm-s-inner .cm-variable { color: rgb(0, 0, 0); }
.cm-s-inner .cm-variable-2 { color: rgb(0, 85, 170); }
.cm-s-inner .cm-variable-3 { color: rgb(0, 136, 85); }
.cm-s-inner .cm-string { color: rgb(170, 17, 17); }
.cm-s-inner .cm-property { color: rgb(0, 0, 0); }
.cm-s-inner .cm-operator { color: rgb(152, 26, 26); }
.cm-s-inner .cm-comment, .cm-s-inner.cm-comment { color: rgb(170, 85, 0); }
.cm-s-inner .cm-string-2 { color: rgb(255, 85, 0); }
.cm-s-inner .cm-meta { color: rgb(85, 85, 85); }
.cm-s-inner .cm-qualifier { color: rgb(85, 85, 85); }
.cm-s-inner .cm-builtin { color: rgb(51, 0, 170); }
.cm-s-inner .cm-bracket { color: rgb(153, 153, 119); }
.cm-s-inner .cm-tag { color: rgb(17, 119, 0); }
.cm-s-inner .cm-attribute { color: rgb(0, 0, 204); }
.cm-s-inner .cm-header, .cm-s-inner.cm-header { color: rgb(0, 0, 255); }
.cm-s-inner .cm-quote, .cm-s-inner.cm-quote { color: rgb(0, 153, 0); }
.cm-s-inner .cm-hr, .cm-s-inner.cm-hr { color: rgb(153, 153, 153); }
.cm-s-inner .cm-link, .cm-s-inner.cm-link { color: rgb(0, 0, 204); }
.cm-negative { color: rgb(221, 68, 68); }
.cm-positive { color: rgb(34, 153, 34); }
.cm-header, .cm-strong { font-weight: 700; }
.cm-del { text-decoration: line-through; }
.cm-em { font-style: italic; }
.cm-link { text-decoration: underline; }
.cm-error { color: red; }
.cm-invalidchar { color: red; }
.cm-constant { color: rgb(38, 139, 210); }
.cm-defined { color: rgb(181, 137, 0); }
div.CodeMirror span.CodeMirror-matchingbracket { color: rgb(0, 255, 0); }
div.CodeMirror span.CodeMirror-nonmatchingbracket { color: rgb(255, 34, 34); }
.cm-s-inner .CodeMirror-activeline-background { background: inherit; }
.CodeMirror { position: relative; overflow: hidden; }
.CodeMirror-scroll { height: 100%; outline: 0px; position: relative; box-sizing: content-box; background: inherit; }
.CodeMirror-sizer { position: relative; }
.CodeMirror-gutter-filler, .CodeMirror-hscrollbar, .CodeMirror-scrollbar-filler, .CodeMirror-vscrollbar { position: absolute; z-index: 6; display: none; }
.CodeMirror-vscrollbar { right: 0px; top: 0px; overflow: hidden; }
.CodeMirror-hscrollbar { bottom: 0px; left: 0px; overflow: hidden; }
.CodeMirror-scrollbar-filler { right: 0px; bottom: 0px; }
.CodeMirror-gutter-filler { left: 0px; bottom: 0px; }
.CodeMirror-gutters { position: absolute; left: 0px; top: 0px; padding-bottom: 30px; z-index: 3; }
.CodeMirror-gutter { white-space: normal; height: 100%; box-sizing: content-box; padding-bottom: 30px; margin-bottom: -32px; display: inline-block; }
.CodeMirror-gutter-wrapper { position: absolute; z-index: 4; background: 0px 0px !important; border: none !important; }
.CodeMirror-gutter-background { position: absolute; top: 0px; bottom: 0px; z-index: 4; }
.CodeMirror-gutter-elt { position: absolute; cursor: default; z-index: 4; }
.CodeMirror-lines { cursor: text; }
.CodeMirror pre { border-radius: 0px; border-width: 0px; background: 0px 0px; font-family: inherit; font-size: inherit; margin: 0px; white-space: pre; overflow-wrap: normal; color: inherit; z-index: 2; position: relative; overflow: visible; }
.CodeMirror-wrap pre { overflow-wrap: break-word; white-space: pre-wrap; word-break: normal; }
.CodeMirror-code pre { border-right: 30px solid transparent; width: fit-content; }
.CodeMirror-wrap .CodeMirror-code pre { border-right: none; width: auto; }
.CodeMirror-linebackground { position: absolute; left: 0px; right: 0px; top: 0px; bottom: 0px; z-index: 0; }
.CodeMirror-linewidget { position: relative; z-index: 2; overflow: auto; }
.CodeMirror-wrap .CodeMirror-scroll { overflow-x: hidden; }
.CodeMirror-measure { position: absolute; width: 100%; height: 0px; overflow: hidden; visibility: hidden; }
.CodeMirror-measure pre { position: static; }
.CodeMirror div.CodeMirror-cursor { position: absolute; visibility: hidden; border-right: none; width: 0px; }
.CodeMirror div.CodeMirror-cursor { visibility: hidden; }
.CodeMirror-focused div.CodeMirror-cursor { visibility: inherit; }
.cm-searching { background: rgba(255, 255, 0, 0.4); }
span.cm-underlined { text-decoration: underline; }
span.cm-strikethrough { text-decoration: line-through; }
.cm-tw-syntaxerror { color: rgb(255, 255, 255); background-color: rgb(153, 0, 0); }
.cm-tw-deleted { text-decoration: line-through; }
.cm-tw-header5 { font-weight: 700; }
.cm-tw-listitem:first-child { padding-left: 10px; }
.cm-tw-box { border-style: solid; border-right-width: 1px; border-bottom-width: 1px; border-left-width: 1px; border-color: inherit; border-top-width: 0px !important; }
.cm-tw-underline { text-decoration: underline; }
@media print {
  .CodeMirror div.CodeMirror-cursor { visibility: hidden; }
}


:root {
    --side-bar-bg-color: #fafafa;
    --control-text-color: #777;
}

@include-when-export url(https://fonts.loli.net/css?family=Open+Sans:400italic,700italic,700,400&subset=latin,latin-ext);

/* open-sans-regular - latin-ext_latin */
  /* open-sans-italic - latin-ext_latin */
    /* open-sans-700 - latin-ext_latin */
    /* open-sans-700italic - latin-ext_latin */
  html {
    font-size: 16px;
}

body {
    font-family: "Open Sans","Clear Sans", "Helvetica Neue", Helvetica, Arial, sans-serif;
    color: rgb(51, 51, 51);
    line-height: 1.6;
}

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</style><title>04内存管理</title>
</head>
<body class='typora-export os-windows'><div class='typora-export-content'>
<div id='write'  class=''><h2 id='四内存管理'><span>四.内存管理</span></h2><h3 id='41-内存布局'><span>4.1 内存布局</span></h3><p><span>iOS的内存整体布局:</span>
<img src="image/0016.jpg" referrerpolicy="no-referrer" alt="alt memory" title="memory"></p><p><span>栈(stack): 方法调用</span>
<span>堆(heap): 通过</span><code>alloc</code><span>、</span><code>copy</code><span>等分配的对象</span>
<span>bss: 未初始化的全局变量</span>
<span>data: 已初始化的全局变量等</span>
<span>text: 程序代码</span></p><p><strong><span>代码区</span></strong><span>：存放程序的二进制代码(即CPU执行的机器指令)，并且是只读的。</span>
<strong><span>数据区</span></strong><span>：分为常量区和静态区。</span></p><ul><li><span>常量区：存放常量(程序在运行的期间不能够被改变的量，例如: 10，字符串常量”abcde”</span></li><li><span>静态区：静态变量和全局变量的存储区域是一起的，一旦静态区的内存被分配, 静态区的内存直到程序全部结束之后才会被释放</span></li></ul><p><strong><span>堆区</span></strong><span>：由程序调用</span><code>malloc()</code><span>函数来主动申请的，需使用</span><code>free()</code><span>函数来释放内存</span>
<strong><span>栈区</span></strong><span>：存放函数内的局部变量，形参和函数返回值。栈区之中的数据的作用范围过了之后，系统就会回收自动管理栈区的内存。</span></p><h3 id='42-内存管理方案'><span>4.2 内存管理方案</span></h3><p><a href='https://www.jianshu.com/p/f683ada6d304' target='_blank' class='url'>https://www.jianshu.com/p/f683ada6d304</a></p><p><span>iOS在内存管理方案，大体有三种：</span></p><ul><li><span>TaggedPointer (一些系统的小对象，比如</span><code>NSNumber</code><span>，为了内存优化，简单理解为披着对象的值类型，其指针的值不再是地址，而是真正的值。)</span></li><li><span>NONPOINTER_ISA (64位架构下的iOS对象方案)</span></li><li><span>散列表(弱引用表 &amp; 引用计数表等)</span></li></ul><p><em><span>TaggedPointer: 已标记的指针</span></em></p><blockquote><p><span>并不是对象的引用计数都直接存储在引用计数表RefcountMap中的， 而是对象的引用计数溢出时，才会移到引用计数表中。</span></p></blockquote><p><span>散列表是</span><code>SidTables()</code><span>结构，包含N个</span><code>SidTable</code><span>结构，每个</span><code>SidTable</code><span>包含三部分:</span></p><ul><li><code>spinlock_t</code><span>  自旋锁，用于上锁/解锁SideTable</span></li><li><code>RefcountMap</code><span>  引用计数表，是一个哈希表</span></li><li><code>Weak_table_t</code><span> 弱引用表，是一个哈希表，是OC中</span><code>weak</code><span>功能实现的核心数据结构</span></li></ul><p><strong><span>A:为什么散列表要设计成包含很多个小</span><code>SidTable</code><span>而不是一个大表？</span></strong>
<span>答: 因为假如都放在一个大表中，当操作一个对象的引用计数时，可能是在不同的线程中，就得给表数据加锁，造成严重的效率问题。对此苹果引用了</span><strong><span>分离锁</span></strong><span>的概念，就可以可以并发操作多个对象的引用计数。</span></p><h3 id='43-数据结构'><span>4.3 数据结构</span></h3><p><strong><span>A.spinlock_t自旋锁</span></strong>
<code>spinlock_t</code><span>自旋锁，是&quot;忙等&quot;的锁，只的是如果当前锁已被其他线程获取，当前线程就会不停探测是否已被释放。自旋锁适用于轻量访问。</span></p><p><em><span>spin: 旋转/转动</span></em></p><p><strong><span>B.引用计数表是通过什么实现的？</span></strong>
<span>通过hash表来实现的。通过hash表来实现的目的是提高查找效率，提高效率的原因是因为插入、和查找都是通过同一个哈希函数得到的内存地址值。</span></p><p><span>备注：</span>
<span>1.</span><code>retainCount</code><span>获取的引用计数值要比对象的真实值多1，最小为1。</span>
<span>2.</span><code>alloc</code><span>的对象引用计数为0。</span></p><p><a href='https://www.jianshu.com/p/17817e6efaf5' target='_blank' class='url'>https://www.jianshu.com/p/17817e6efaf5</a>
<a href='https://www.jianshu.com/p/0ecd91dc34aa' target='_blank' class='url'>https://www.jianshu.com/p/0ecd91dc34aa</a></p><h3 id='44-arc和mrc'><span>4.4 ARC和MRC</span></h3><p><span>MRC：手动引用计数</span>
<span>ARC：自动引用计数</span>
<span>ARC是编译器LLVM(编译器)在合适的位置，自动为我们插入</span><code>retain</code><span>和</span><code>release</code><span>操作，还需要和Runtime共同协作的结果。</span></p><p><em><span>retain：保持,拥有</span></em></p><h3 id='45-引用计数'><span>4.5 引用计数</span></h3><p><code>alloc</code><span>实现:</span>
<span>经过一系列调用，最终调用了C函数的</span><code>calloc</code><span>.此时并没有设置引用计数为1.为什么此时调用</span><code>retainCount</code><span>返回1，是因为获取引用计数时，源码显示，变量初始化为了1。</span></p><p><code>retain</code><span>实现：</span>
<span>找到</span><code>SideTables()</code><span>中引用计数的表（2次哈希查找），增加1，并重写赋值到</span><code>SideTables()</code></p><p><code>release</code><span>实现：</span>
<span>和</span><code>retain</code><span>正好相反。</span></p><p><code>dealloc</code><span>实现：</span>
<img src="image/0017.jpg" referrerpolicy="no-referrer" alt="alt dealloc" title="dealloc"></p><p><em><span>备注，对象在哪个线程销毁的，就会在哪个线程调用dealloc，并不一定是在主线程dealloc</span></em></p><p><strong><span>A.当为使用关联对象的技术给类添加实例变量，那么在对象的dealloc方法中，是否有必要对它的关联对象进行移除操作?</span></strong>
<span>答: 不需要，因为从dealloc内部源码中可以得知，它会自动判断对象是否有关联对象，有的话就会调用移除操作，所以不需要程序员手动移除。</span></p><p><strong><span>B.为什么weak属性，会在引用计数为0的时候，自动置为nil?</span></strong>
<span>答: 当对象被废弃之后，dealloc方法的内部实现中，会调用清除弱引用的方法，而在这个清除弱引用的方法中，会通过哈希算法，来查找被废弃对象在弱引用表中的位置，来提取它的弱引用数据表，然后进行for循环遍历，并置为nil.</span>
<span>因为从类dealloc内部源码中可以得知，它会判断对象是否有弱引用指针，有的话，会将指向该对象的弱引用指针置为nil.置为nil的核心函数是</span><code>weak_clear_no_lock()</code></p><p><strong><span>C.alloc创建的对象，它的引用计数为多少?</span></strong></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">// 这个NSObject对象的引用计数是多少？</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">NSObject *obj = [[NSObject alloc] init];</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 46px;"></div><div class="CodeMirror-gutters" style="display: none; height: 46px;"></div></div></div></pre><p><span>答：真实的引用计数是0，但是如果打印引用计数会是1，为什么此时调用</span><code>retainCount</code><span>返回1，是因为获取引用计数时，源码显示，变量初始化为了1。</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c++"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c++"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><span><span>​</span>x</span></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">objc_object::sidetable_retainCount</span>() {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">SideTable</span><span class="cm-operator">&amp;</span> <span class="cm-variable">table</span> <span class="cm-operator">=</span> <span class="cm-variable">SideTables</span>()[<span class="cm-keyword">this</span>];</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable-3">size_t</span> <span class="cm-variable">refcnt_result</span> <span class="cm-operator">=</span> <span class="cm-number">1</span>; &nbsp; <span class="cm-comment">// 初始化是1</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">table</span>.<span class="cm-variable">lock</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-comment">// 省略部分代码...</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">table</span>.<span class="cm-variable">unlock</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">return</span> <span class="cm-variable">refcnt_result</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 230px;"></div><div class="CodeMirror-gutters" style="display: none; height: 230px;"></div></div></div></pre><h3 id='46-弱引用'><span>4.6 弱引用 </span></h3><p><span>一个被声明为</span><code>__weak</code><span>的对象指针，经过编译器的编译之后，会调用</span><code>objc_initWeak()</code><span>方法，再经过一系列的函数调用栈，最终添加到弱引用表中，具体的添加的位置，是通过哈希算法进行查找的。</span></p><ul><li><span>用</span><code>weak</code><span>修饰的对象在销毁后会被自动置为</span><code>nil</code></li><li><span>凡是用</span><code>weak</code><span>修饰过的对象,必定是注册到</span><code>autoreleasepool</code><span>中的对象</span></li></ul><blockquote><p><span>作为对比：被</span><code>retain</code><span>修饰的对象在销毁后，不会自动置为</span><code>nil</code></p></blockquote><p><strong><span>A.weak表(弱引用表)中的对象是如何被释放的呢?</span></strong>
<span>1.从</span><code>weak</code><span>表中获取废弃对象的键值记录.</span>
<span>2.将记录中所有包含</span><code>__weak</code><span>的变量地址,赋值为</span><code>nil</code><span>.</span>
<span>3.从</span><code>weak</code><span>表中删除该记录.</span>
<span>4.从引用计数表中删除对应的记录.</span></p><p><span>这就是</span><code>__weak</code><span>修饰的变量会在释放后自动置为</span><code>nil</code><span>的原因.同时,因为</span><code>weak</code><span>修饰之后涉及到注册到</span><code>weak</code><span>表等相关操作,如果大量使用</span><code>weak</code><span>可能会造成不必要的CPU资源浪费, 所以尽量不要在循环引用中使用</span><code>weak</code><span>.</span></p><p><span>这里不得不提到另外一个和</span><code>__weak</code><span>相近的属性:</span><code>__unsafe_unretained</code><span>,它与</span><code>weak</code><span>的区别在于,释放对象后不会对其置为</span><code>nil</code><span>.</span></p><p><strong><span>B.为什么有循环引用block内用</span><code>weakObject</code><span>的时候最好能在block内套一层</span><code>strongObject</code><span>?</span></strong>
<span>答：1.在异步线程中</span><code>weakObject</code><span>可能会被销毁,所以需要套一层</span><code>strong</code><span>来持有对象；(比如block中需要分步骤对对象某属性进行+1-1操作，可能-1后，对象就释放了，那么就会造成+1不会执行了)</span>
<span>2.如果内部有耗时的循环语句,频繁使用</span><code>weakObject</code><span>也会增加内存损耗，这牵涉到weak对象会被加入</span><code>autoreleasepool</code><span>，和弱引用表的查找增删有关系。</span></p><p><strong><span>C.</span><code>__weak</code><span>在循环中的内存问题？</span></strong>
<span>在runtime源码中，关于</span><code>__Weak</code><span>的核心代码是:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c++"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c++"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">/** </span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-comment">* This loads the object referenced by a weak pointer and returns it, after</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-comment">* retaining and autoreleasing the object to ensure that it stays alive</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-comment">* long enough for the caller to use it. This function would be used</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-comment">* anywhere a __weak variable is used in an expression.</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-comment">*/</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">id</span> <span class="cm-def">objc_loadWeak</span>(<span class="cm-variable">id</span> <span class="cm-operator">*</span><span class="cm-variable">location</span>) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">if</span> (<span class="cm-operator">!*</span><span class="cm-variable">location</span>) <span class="cm-keyword">return</span> <span class="cm-variable">nil</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-keyword">return</span> <span class="cm-variable">objc_autorelease</span>(<span class="cm-variable">objc_loadWeakRetained</span>(<span class="cm-variable">location</span>));</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 230px;"></div><div class="CodeMirror-gutters" style="display: none; height: 230px;"></div></div></div></pre><p><span>阅读</span><code>objc_loadWeakRetained()</code><span>函数之后，可以得出下面的结论：</span></p><p><span>每使用一次</span><code>__weak</code><span>对象，运行时系统都会将其指向的原始对象先</span><code>retain</code><span>，之后保存到自动释放池中（ </span><code>AutoReleasePoolPage</code><span>的</span><code>add()</code><span>函数）。因此如果大量调用</span><code>__weak</code><span>对象，则会重复进行此工作。不仅耗费无意义的性能（重复存储同一对象），还会使内存在短时间内大量增长。</span></p><p><span>也就是有下面2个重要的结论：</span></p><ul><li><span>使用附有</span><code>__weak</code><span>修饰符的变量，即是使用注册到a</span><code>utoreleasepool</code><span>中的对象</span></li><li><span>如果大量地使用附有</span><code>__weak</code><span>修饰符的变量，注册到</span><code>autoreleasepool</code><span>的对象也会大量增加 </span><em><span>(别在循环中多次使用</span><code>__weak</code><span>)</span></em></li></ul><h3 id='47-自动释放池'><span>4.7 自动释放池</span></h3><p><strong><span>A.AutoreleasePool的实现原理是怎么的？</span></strong>
<code>AutoreleasePool</code><span>是以栈为结点通过双向链表的形式组合而成，并且是和线程一一对应的。</span></p><p><code>@autoreleasepool{}</code><span>被编译器编译后，代码如下：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">// push</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">void</span> <span class="cm-variable-3">*</span><span class="cm-variable">poolToken</span> <span class="cm-operator">=</span> <span class="cm-variable">objc_autoreleasePoolPush</span>();</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">// 这中间为写在{...}中的代码</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment">// pop</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">objc_autoreleasePoolPop</span>(<span class="cm-variable">poolToken</span>);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 115px;"></div><div class="CodeMirror-gutters" style="display: none; height: 115px;"></div></div></div></pre><p><span>而这两个函数都是对</span><code>AutoreleasePoolPage</code><span>的简单封装，所以自动释放机制的核心就在于这个类：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">class</span> <span class="cm-variable">AutoreleasePoolPage</span> {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">id</span> <span class="cm-operator">*</span><span class="cm-variable">next</span>; &nbsp;<span class="cm-comment">// 栈中下一个可填充的位置</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">pthread_t</span> <span class="cm-keyword">const</span> <span class="cm-variable">thread</span>; &nbsp;<span class="cm-comment">//  指向当前线程；</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">AutoreleasePoolPage</span> <span class="cm-operator">*</span> <span class="cm-keyword">const</span> <span class="cm-variable">parent</span>; &nbsp;<span class="cm-comment">// 指向父结点,第一个parent节点为nil</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">AutoreleasePoolPage</span> <span class="cm-operator">*</span><span class="cm-variable">child</span>; &nbsp;<span class="cm-comment">// 指向子结点,最后一个child节点为nil</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 138px;"></div><div class="CodeMirror-gutters" style="display: none; height: 138px;"></div></div></div></pre><p><code>AutoreleasePool</code><span>并没有单独的结构，而是由若干个</span><code>AutoreleasePoolPage</code><span>以双向链表的形式组合而成。每创建一个池子，会在首部创建一个</span><code>哨兵</code><span>对象,作为标记，最外层池子的顶端会有一个next指针。当链表容量满了，就会在链表的顶端，并指向下一张表。</span></p><p><span>总结：</span></p><ul><li><code>AutoreleasePoolPage</code><span>是以栈为结点通过双向链表的形式组合而成；遵循先进后出规则，整个自动释放池由一系列的</span><code>AutoreleasePoolPage</code><span>组成的，而</span><code>AutoreleasePoolPage</code><span>是以双向链表的形式连接起来。</span></li><li><span>自动释放池与线程一一对应</span></li><li><code>POOL_BOUNDARY</code><span>为哨兵对象，入栈时插入，出栈时释放对象到此传入的哨兵对象</span></li></ul><p><strong><span>A1.源码发现的：</span></strong></p><p><img src="image/0057.jpeg" referrerpolicy="no-referrer" alt="alt AutoreleasePool" title="AutoreleasePool"></p><p><span>在objc源码中，在文件</span><code>NSObject-internal.h</code><span>文件中有对自动释放池的简单讲解：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">Autorelease</span> <span class="cm-variable">pool</span> <span class="cm-variable">implementation</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> <span class="cm-variable">thread</span><span class="cm-string">'s autorelease pool is a stack of pointers.</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">Each</span> <span class="cm-variable">pointer</span> <span class="cm-variable">is</span> <span class="cm-variable">either</span> <span class="cm-variable">an</span> <span class="cm-variable">object</span> <span class="cm-variable">to</span> <span class="cm-variable">release</span>, <span class="cm-variable">or</span> <span class="cm-variable">POOL_BOUNDARY</span> <span class="cm-variable">which</span> <span class="cm-variable">is</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">an</span> <span class="cm-variable">autorelease</span> <span class="cm-variable">pool</span> <span class="cm-variable">boundary</span>.</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">A</span> <span class="cm-variable">pool</span> <span class="cm-variable">token</span> <span class="cm-variable">is</span> <span class="cm-variable">a</span> <span class="cm-variable">pointer</span> <span class="cm-variable">to</span> <span class="cm-variable">the</span> <span class="cm-variable">POOL_BOUNDARY</span> <span class="cm-keyword">for</span> <span class="cm-variable">that</span> <span class="cm-variable">pool</span>. <span class="cm-variable">When</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">the</span> <span class="cm-variable">pool</span> <span class="cm-variable">is</span> <span class="cm-variable">popped</span>, <span class="cm-variable">every</span> <span class="cm-variable">object</span> <span class="cm-variable">hotter</span> <span class="cm-variable">than</span> <span class="cm-variable">the</span> <span class="cm-variable">sentinel</span> <span class="cm-variable">is</span> <span class="cm-variable">released</span>.</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">The</span> <span class="cm-variable">stack</span> <span class="cm-variable">is</span> <span class="cm-variable">divided</span> <span class="cm-variable">into</span> <span class="cm-variable">a</span> <span class="cm-variable">doubly</span><span class="cm-operator">-</span><span class="cm-variable">linked</span> <span class="cm-variable">list</span> <span class="cm-variable">of</span> <span class="cm-variable">pages</span>. <span class="cm-variable">Pages</span> <span class="cm-variable">are</span> <span class="cm-variable">added</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">and</span> <span class="cm-variable">deleted</span> <span class="cm-variable">as</span> <span class="cm-variable">necessary</span>.</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">Thread</span><span class="cm-operator">-</span><span class="cm-variable">local</span> <span class="cm-variable">storage</span> <span class="cm-variable">points</span> <span class="cm-variable">to</span> <span class="cm-variable">the</span> <span class="cm-variable">hot</span> <span class="cm-variable">page</span>, <span class="cm-variable">where</span> <span class="cm-variable">newly</span> <span class="cm-variable">autoreleased</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> <span class="cm-variable">objects</span> <span class="cm-variable">are</span> <span class="cm-variable">stored</span>.</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 253px;"></div><div class="CodeMirror-gutters" style="display: none; height: 253px;"></div></div></div></pre><p><span>哨兵对象</span><code>POOL_BOUNDARY</code><span>其实就是一个</span><code>nil</code><span>，用来区分不同的自动释放池，以解决自动释放池嵌套的问题。</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang=""><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang=""><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">#define POOL_BOUNDARY nil</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre><p><strong><span>B.AutoreleasePool为何可以嵌套使用？</span></strong>
<code>POOL_BOUNDARY</code><span>用来区分不同的自动释放池，以解决自动释放池嵌套的问题：</span>
<span>1.每当创建一个自动释放池，就会调用</span><code>push()</code><span>方法将一个</span><code>POOL_BOUNDARY</code><span>入栈，并返回其存放的内存地址；</span>
<span>2.当往自动释放池中添加</span><code>autorelease</code><span>对象时，将</span><code>autorelease</code><span>对象的内存地址入栈，它们前面至少有一个</span><code>POOL_BOUNDARY</code><span>；</span>
<span>3.当销毁一个自动释放池时，会调用</span><code>pop()</code><span>方法并传入一个</span><code>POOL_BOUNDARY</code><span>，会从自动释放池中最后一个对象开始，依次给它们发送</span><code>release</code><span>消息，直到遇到这个</span><code>POOL_BOUNDARY</code><span>。</span></p><p><span>所以这个是</span><code>AutoreleasePool</code><span>的底层原理有关系，牵涉到C++里面哨兵对象的概念，多次嵌套使用，反应到内存结构上，其实就是多次插入哨兵对象，所以是可以嵌套使用的。</span></p><p><strong><span>C.下面代码中的Array是什么时候释放的？</span></strong></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-operator">-</span> (<span class="cm-variable-3">void</span>)<span class="cm-variable">viewDidLoad</span> {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp;  [<span class="cm-variable">super</span> <span class="cm-variable">viewDidLoad</span>];</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">NSMutableArray</span> <span class="cm-operator">*</span><span class="cm-variable">myArr</span> <span class="cm-operator">=</span> [<span class="cm-variable">NSMutableArray</span> <span class="cm-variable">array</span>];</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 92px;"></div><div class="CodeMirror-gutters" style="display: none; height: 92px;"></div></div></div></pre><p><span>从代码角度来说，执行完这个代码块之后，变量就会被销毁。从内存角度来说，在每次runloop将要结束的时候，都会调用</span><code>AutoreleasePoolPage的pop()</code><span>方法，对已无用的对象进行一次</span><code>release</code><span>操作，进而释放对象。</span></p><p><strong><span>D.AutoreleasePool的应用场景？</span></strong>
<span>比如在for循环中alloc图片数据等内存消耗较大的场景，或者有大量临时变量等，可以手动插入AutoreleasePool来降低内存的峰值</span></p><p><strong><span>E.NSThread、NSRunLoop 和 NSAutoreleasePool三者之间的关系</span></strong></p><ul><li><code>NSThread</code><span>和</span><code>NSRunLoop</code><span>是一一对应的关系</span></li><li><span>在</span><code>NSRunLoop</code><span>对象的每个运行循环（event loop）开始前，系统会自动创建一个</span><code>autoreleasepool</code><span>，并在运行循环（event loop）结束时</span><code>drain</code><span>掉这个pool，同时释放所有</span><code>autoreleased</code><span>对象</span></li><li><code>autoreleasepool</code><span>只会对应一个线程，每个线程可能会对应多个</span><code>autoreleasepool</code><span>，比如</span><code>autoreleasepool</code><span>嵌套的情况</span></li></ul><p><em><span>drain: 排空/消耗/耗尽</span></em></p><p><strong><span>F.Autorelease对象什么时候释放？</span></strong>
<span>在没有手加</span><code>AutoreleasePool</code><span>的情况下，</span><code>Autorelease</code><span>对象是在当前的runloop循环结束时释放的，而它能够释放的原因是系统在每个runloop循环中都加入了自动释放池Push和Pop.</span></p><h3 id='48-循环引用'><span>4.8 循环引用</span></h3><p><span>三种循环引用：</span></p><ul><li><span>自循环引用</span></li><li><span>相互循环引用</span></li><li><span>多循环引用</span></li></ul><p><span>破除循环引用的两个思路：</span></p><ul><li><span>避免产生循环引用</span></li><li><span>在合适的时机手动断环</span></li></ul><p><span>接触循环引用的办法(三个)：</span><code>__weak</code><span>, </span><code>__block</code><span>, </span><code>__unsafe_unretained</code></p><p><code>__block</code><span>的注意点：</span>
<span>在</span><strong><span>MRC</span></strong><span>下，</span><code>__block</code><span>修饰对象，不会增加其引用计数，避免了循环引用</span>
<span>在</span><strong><span>ARC</span></strong><span>下，</span><code>__block</code><span>修饰对象，会被强引用，无法避免循环引用，需要手动解环</span></p><p><code>__unsafe_unretained</code><span>的注意点：</span>
<span>修饰对象不会增加引用计数，但是对象如果被释放了，会产生</span><strong><span>悬垂指针</span></strong><span>。</span></p><p><strong><span>A.在日常开发中，是否遇到了循环引用，又是如何解决的？</span></strong>
<span>答：循环引用大体可以分为三种情况：自循环引用、相互循环引用、多循环引用，而解决循环引用是有2个思路：一是避免循环引用的产生、二是在合适的时机手动断环。比如在开发中，我遇到的</span><code>NSTimer</code><span>的循环引用情况，由于runloop是强引用</span><code>NSTimer</code><span>的，所以</span><code>NSTimer</code><span>非常容易和对象循环引用，而解决</span><code>NSTimer</code><span>的循环引用，是创建了一个中间对象，利用中间对象持有</span><code>NSTimer</code><span>和目标对象的弱引用，在</span><code>NSTimer</code><span>的回调里，进行逻辑判断，并在合适的时机取消关闭</span><code>NSTimer</code><span>.</span></p><p><strong><span>B.为什么a不会影响结果？b却影响结果？</span></strong></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="objc"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="objc"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">int</span> <span class="cm-variable">a</span> <span class="cm-operator">=</span> <span class="cm-number">10</span>; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; <span class="cm-comment">// 普通变量</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">__block</span> <span class="cm-variable-3">int</span> <span class="cm-variable">b</span> <span class="cm-operator">=</span> <span class="cm-number">20</span>; &nbsp; <span class="cm-comment">// 带__block修饰符的block普通变量</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable-3">void</span> (<span class="cm-variable">^testBlock</span>)(<span class="cm-variable-3">int</span>) <span class="cm-operator">=</span> <span class="cm-variable">^</span>(<span class="cm-variable-3">int</span> <span class="cm-variable">c</span>) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable-3">int</span> <span class="cm-variable">d</span> <span class="cm-operator">=</span> <span class="cm-variable">a</span> <span class="cm-operator">+</span> <span class="cm-variable">b</span> <span class="cm-operator">+</span> <span class="cm-variable">c</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">NSLog</span>(<span class="cm-keyword">@</span><span class="cm-string">"%d"</span>, <span class="cm-variable">d</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">};</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">a</span> <span class="cm-operator">=</span> <span class="cm-number">20</span>; &nbsp; &nbsp; &nbsp;<span class="cm-comment">// 修改值不会影响testBlock内的计算结果</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">b</span> <span class="cm-operator">=</span> <span class="cm-number">30</span>; &nbsp; &nbsp; &nbsp;<span class="cm-comment">// 修改值会影响testBlock内的计算结果</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">testBlock</span>(<span class="cm-number">1</span>);</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 253px;"></div><div class="CodeMirror-gutters" style="display: none; height: 253px;"></div></div></div></pre><p><span>答：被</span><code>__block</code><span>修饰的变量，编译之后，会被包装成</span><code>__block_impl_x</code><span>的结构体，而Block内部会持有这个结构体的</span><code>__forwarding</code><span>指针地址，所以Block在修改这个变量时，Block会通过引用获取到最新的值，而没有被</span><code>__block</code><span>修饰的变量，会直接传入block内部并复制。</span></p><p><span>结构体代码如下：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">struct</span> <span class="cm-def">__block_impl</span> {</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable-3">void</span> <span class="cm-variable-3">*</span><span class="cm-variable">__isa</span>; &nbsp;<span class="cm-comment">// 指针</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-comment">// 关键，传入变量的地址，也是为什么可以改变__block修饰的变量的原因</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">__Block_byref_b_0</span> <span class="cm-operator">*</span><span class="cm-variable">__forwarding</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable-3">int</span> <span class="cm-variable">__size</span>; &nbsp;<span class="cm-comment">// 结构体大小</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable-3">int</span> <span class="cm-variable">b</span>; &nbsp;<span class="cm-comment">// 真正的值</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">}</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 161px;"></div><div class="CodeMirror-gutters" style="display: none; height: 161px;"></div></div></div></pre><p><span>所以，没有</span><code>__block</code><span>修饰的变量和有</span><code>__block</code><span>修饰的变量在调用时的代码为:</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="c"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="c"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">a</span> <span class="cm-operator">=</span> <span class="cm-number">20</span>;</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">(<span class="cm-variable">b</span>.<span class="cm-variable">__forwarding</span><span class="cm-operator">-&gt;</span><span class="cm-variable">b</span>) <span class="cm-operator">=</span> <span class="cm-number">40</span>;</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 46px;"></div><div class="CodeMirror-gutters" style="display: none; height: 46px;"></div></div></div></pre><p><strong><span>C.使用</span><code>__block</code><span>手动解环?</span></strong>
<span>注意，ARC下才需要这种做。</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="objc"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="objc"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation" style=""><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">typedef</span> <span class="cm-variable-3">void</span>(<span class="cm-variable">^TTBlock</span>)(<span class="cm-variable-3">int</span>); &nbsp;<span class="cm-comment">// 定义</span></span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">@property</span> (<span class="cm-keyword">nonatomic</span> ,<span class="cm-keyword">copy</span>) <span class="cm-variable">TTBlock</span> <span class="cm-variable">myBlock</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-variable">__block</span> <span class="cm-variable">ViewController</span> <span class="cm-operator">*</span><span class="cm-variable">blockSelf</span> <span class="cm-operator">=</span> <span class="cm-keyword">self</span>;</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span cm-text="">​</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-keyword">self</span>.<span class="cm-variable">myBlock</span> <span class="cm-operator">=</span> <span class="cm-variable">^</span>(<span class="cm-variable-3">int</span> <span class="cm-variable">c</span>) {</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">NSLog</span>(<span class="cm-keyword">@</span><span class="cm-string">"%ld"</span>, <span class="cm-variable">blockSelf</span>.<span class="cm-variable">hash</span>);</span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"> &nbsp; &nbsp;<span class="cm-variable">blockSelf</span> <span class="cm-operator">=</span> <span class="cm-keyword">nil</span>; &nbsp;<span class="cm-comment">// 手动去掉环</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;">};</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 207px;"></div><div class="CodeMirror-gutters" style="display: none; height: 207px;"></div></div></div></pre><p><strong><span>D.编译OC代码成C++代码文件?</span></strong>
<span>将OC编译成C++可以查看一些OC实现原理。首先需要知道：</span></p><ul><li><span>Objective-C文件的后缀名是 </span><code>.h</code><span> 和 </span><code>.m</code></li><li><span>C语言的后缀名是</span><code>.h</code><span> 和 </span><code>.c</code></li><li><span>C++的后缀名是</span><code>.h</code><span> 和 </span><code>.cpp</code><span> </span></li><li><span>混合代码后缀名是 </span><code>.h</code><span> 和 </span><code>.mm</code></li></ul><p><span>首先cd到代码目录，进行编译：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded" spellcheck="false" lang="shell"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="shell"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"><pre><span>xxxxxxxxxx</span></pre></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-def">$ clang</span> <span class="cm-attribute">-rewrite-objc</span> main.m</span></pre></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-comment"># arc环境可以增加如下参数:</span></span></pre><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-def">$ clang</span> <span class="cm-attribute">-rewrite-objc</span> <span class="cm-attribute">-fobjc-arc</span> main.m</span></pre></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 69px;"></div><div class="CodeMirror-gutters" style="display: none; height: 69px;"></div></div></div></pre><p><span>如果想编译</span><code>Controller.m</code><span>等之类文件，需要一些编译环境和库的参数配置：</span></p><pre class="md-fences md-end-block ty-contain-cm modeLoaded md-focus" spellcheck="false" lang="shell"><div class="CodeMirror cm-s-inner CodeMirror-wrap" lang="shell"><div style="overflow: hidden; position: relative; width: 3px; height: 0px; top: 9.5px; left: 8px;"><textarea autocorrect="off" autocapitalize="off" spellcheck="false" tabindex="0" style="position: absolute; bottom: -1em; padding: 0px; width: 1000px; height: 1em; outline: none;"></textarea></div><div class="CodeMirror-scrollbar-filler" cm-not-content="true"></div><div class="CodeMirror-gutter-filler" cm-not-content="true"></div><div class="CodeMirror-scroll" tabindex="-1"><div class="CodeMirror-sizer" style="margin-left: 0px; margin-bottom: 0px; border-right-width: 0px; padding-right: 0px; padding-bottom: 0px;"><div style="position: relative; top: 0px;"><div class="CodeMirror-lines" role="presentation"><div role="presentation" style="position: relative; outline: none;"><div class="CodeMirror-measure"></div><div class="CodeMirror-measure"></div><div style="position: relative; z-index: 1;"></div><div class="CodeMirror-code" role="presentation"><div class="CodeMirror-activeline" style="position: relative;"><div class="CodeMirror-activeline-background CodeMirror-linebackground"></div><div class="CodeMirror-gutter-background CodeMirror-activeline-gutter" style="left: 0px; width: 0px;"></div><pre class=" CodeMirror-line " role="presentation"><span role="presentation" style="padding-right: 0.1px;"><span class="cm-def">$ xcrun</span> <span class="cm-attribute">-sdk</span> iphonesimulator clang <span class="cm-attribute">-rewrite-objc</span> ViewController.m</span></pre></div></div></div></div></div></div><div style="position: absolute; height: 0px; width: 1px; border-bottom: 0px solid transparent; top: 23px;"></div><div class="CodeMirror-gutters" style="display: none; height: 23px;"></div></div></div></pre></div></div>
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